Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. Pat. No. 7,832,207, filed Apr. 9, 2009 (the '207 patent) and U.S. Pat. No. 7,874,155, filed Feb. 25, 2010 (the '155 patent), the disclosures of which are hereby incorporated herein by reference in their entireties. The '207 and '155 patents disclose systems and techniques for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and techniques for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '207 and '155 patents are shown and described in U.S. patent application Ser. No. 12/879,595, filed Sep. 10, 2010 (the '595 application), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '207 and '155 patents, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in the '595 application as well as in U.S. patent application Ser. No. 12/938,853, filed Nov. 3, 2010 (the '853 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
Similarly, the most common form of internal combustion engine, found in automotive, aviation, marine, generation, and other applications, typically contains multiple cylinders in which expanding gases drive pistons. These pistons are linked to a common crankshaft by mechanisms that require precise synchronization among cylinders: in some cases, loss of synchronization (e.g., by failure of a timing belt in an automotive engine) can destroy the mechanism. To transfer power from the crankshaft to a mechanical load, it is often necessary to interpose a gearbox, infinitely variable transmission, or other device to convert the crankshaft's working range of rotational speeds to another more suitable range. The spatial arrangement of cylinders around the crankshaft tends to be strongly constrained by the need to balance forces, minimize vibration, and manufacture identical parts for the sake of economy. Moreover, such an engine typically burns fuel near peak efficiency over a relatively small range of cyclic frequencies, so that some states of operation (e.g., acceleration) are low-efficiency, high-pollution.
One alternative to mechanical coupling of piston-type cylinders to a shaft (e.g., by a linkage composed of rods, cranks, and other rigid parts) is hydraulic coupling. In hydraulic coupling, the work performed by expanding gases within a piston-type cylinder is not transmitted to a load as mechanical force acting through the rigid parts of a mechanical linkage, but as pressure in a hydraulic fluid. This fluid may drive a hydraulic motor whose output is torque applied to a rotating shaft analogous to a crankshaft. Hydraulic devices offer several advantages in such a role, including, for example, adjustable and efficient operability over a range of flow rates, reversibility, arbitrary spatial orientation, arbitrary spatial location (within limits).
However, energy conversion or storage systems that feature hydraulic motor/pumps may suffer from parasitic energy losses entailed by valve actuation and throttling or by the driving of hydraulic motor/pumps over non-optimal fluid pressure ranges (i.e., input pressure for motors or for pump/motors operated as motors, output pressure for pumps or for pump/motors operated as pumps). Reduction or elimination of such losses would increase the overall efficiency of such energy conversion or storage systems. For example, overall efficiency may be increased by limiting the hydraulic pressure range delivered to a hydraulic motor, even for broad ranges of pressures experienced in the system during internal combustion or the expansion and/or compression of compressed gas.